This invention relates generally to electrical communication and particularly to mobile radio communication and even more particularly to code division multiple access in such communication.
Modern communication systems, such as cellular and satellite radio systems, employ various modes of operation (analog, digital, and hybrids) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In a typical direct sequence CDMA (DS-CDMA) system, an information bit stream to be transmitted is superimposed on a much-higher-rate bit stream that typically consists of consecutive symbols that are sometimes called spreading sequences. Each bit of a spreading sequence is commonly called a chip. Usually, each information bit stream is allocated a unique spreading sequence that is consecutively repeated to form the much-higher-rate bit stream. Each bit of the information bit stream and the spreading sequence are typically combined by multiplication, or modulo-2 addition, in a process sometimes called coding or spreading the information signal. The combined bits stream may be scrambled by multiplication by another, usually pseudo-noise, bit stream, with the result transmitted as a modulation of a carrier wave. A receiver demodulates the modulated carrier and correlates the resulting signal with the scrambling bit stream and the unique spreading sequence to recover the information bit stream that was transmitted.
Digital cellular communication systems have expanded functionality for optimizing system capacity and supporting hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term "macrocell" generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms "microcell" and "picocell" generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, e.g., a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building. From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments.
FIG. 1 illustrates an exemplary hierarchical, or multi-layered, cellular system. An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure. Each umbrella cell may contain an underlying microcell structure. The umbrella cell 10 includes microcell 20 represented by the area enclosed within the dotted line and microcell 30 represented by the area enclosed within the dashed line corresponding to areas along city streets, and picocells 40, 50, and 60, which cover individual floors of a building. The intersection of the two city streets covered by the microcells 20 and 30 may be an area of dense traffic concentration, and thus might represent a hot spot.
FIG. 2 is a block diagram of an exemplary cellular mobile radiotelephone system, including an exemplary base station (BS) 110 and mobile station (MS) 120. The BS includes a control and processing unit 130 which is connected to a mobile switching center (MSC) 140 which in turn is connected to the public switched telephone network (PSTN) (not shown). General aspects of such cellular radiotelephone systems are known in the art. The BS 110 handles a plurality of traffic channels, which may carry voice, facsimile, video, and other information, through a traffic channel transceiver 150, which is controlled by the control and processing unit 130. Also, each BS includes a control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the BS or cell to MSs locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the traffic and control transceiver 170, for use with control and traffic channels that share the same radio carrier.
The MS 120 receives the information broadcast on a control channel at its traffic and control channel transceiver 170. Then, the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the MS to lock on to, and determines on which cell the MS should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated, as described for example in U.S. Pat. No. 5,353,332 to Raith et al., entitled "Method and Apparatus for Communication Control in a Radiotelephone System".
In North America, a digital cellular radiotelephone system using TDMA is called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using DS-CDMA is specified by the TIA/EIA/IS-95 standard, and a frequency hopping CDMA communication system is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the Intentional Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), and Japan's Association of Radio Industries and Businesses (ARIB). Besides transmitting voice information, the next generation systems can be expected to carry packet data and to inter-operate with packet data networks that are also usually designed and based on industry-wide data standards such as the open system interface (OSI) model or the transmission control protocol/Internet protocol (TCP/IP) stack. These standards have been developed, whether formally or de facto, for many years, and the applications that use these protocols are readily available. The main objective of standards-based networks is to achieve interconnectivity with other networks. The Internet is today's most obvious example of such a standards-based packet data network in pursuit of this goal.
In most of these digital communication systems, communication channels are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz), 900 MHz, 1800 MHz, and 1900 MHz. In TDMA systems and even to varying extents in CDMA systems, each radio channel is divided into a series of time slots, each of which contains a block of information from a user. The time slots are grouped into successive frames that each have a predetermined duration, and successive frames may be grouped into a succession of what are usually called superframes. The kind of access technique (e.g., TDMA or CDMA) used by a communication system affects how user information is represented in the slots and frames, but current access techniques all use a slot/frame structure.
Time slots assigned to the same user, which may not be consecutive time slots on the radio carrier, may be considered a logical channel assigned to the user. During each time slot, a predetermined number of digital bits are transmitted according to the particular access technique (e.g., CDMA) used by the system. In addition to logical channels for voice or data traffic, cellular radio communication systems also provide logical channels for control messages, such as paging/access channels for call-setup messages exchanged by BSs and MSs and synchronization channels for broadcast messages used by MSs and other remote terminals for synchronizing their transceivers to the frame/slot/bit structures of the BSs. In general, the transmission bit rates of these different channels need not coincide and the lengths of the slots in the different channels need not be uniform. Moreover, third generation cellular communication systems being considered in Europe and Japan are asynchronous, meaning that the structure of one BS is not temporally related to the structure of another BS and that an MS does not know any of the structures in advance.
FIG. 3 illustrates a radio frame that includes a number of complex (in-phase and quadrature) chips divided among sixteen slots. The radio frame may have a duration of ten milliseconds (10 ms) and include 40960 chips. Each slot thus includes 2560 chips, which may represent ten 256-chip symbols. Such a frame/slot/chip structure is a feature of a third generation, wideband CDMA communication system under consideration by ETSI. The radio signal transmitted by a BS in such a communication system is the sum of spread and scrambled data and control bits and an unscrambled synchronization channel. Data and control bits are typically spread by either bit-wise (DS-CDMA) or block-wise replacement by an orthogonal sequence or sequences, such as Walsh-Hadamard sequences. (This is sometimes called m-ary orthogonal keying.) As noted above, the spread results are then scrambled usually by bit-wise modulo-2 addition of a pseudo-noise (PN) scrambling sequence.
It will be appreciated that the data bits include user information, such as audio, video, and text information, and that the information of different users is made distinguishable, in accordance with CDMA principles, by using distinguishable spreading sequences, such as mutually orthogonal Walsh-Hadamard sequences. In a sense, then, each user's Walsh-Hadamard sequence(s) define that user's communication channel, and thus these distinguishable sequences are said to channelize the user information. The construction of sequences according to their correlation properties is described in U.S. Pat. No. 5,353,352 to P. Dent et al. for "Multiple Access Coding for Radio Communications" and U.S. Pat. No. 5,550,809 to G. Bottomley et al. for "Multiple Access Coding Using Bent Sequences for Mobile Radio Communications". These patents are expressly incorporated here by reference.
In conventional CDMA communication systems, each Walsh-Hadamard sequence is a row of an MxM Walsh-Hadamard matrix H.sub.M, and the entries in H.sub.M (the components of the sequences) are either +1 or -1. The matrix H.sub.M is generated in the usual way according to the following expression: ##EQU1## with H.sub.1 =[+1] or [-1].
One of the advantages of Walsh-Hadamard sequences for channelization is that user information in a received signal can be efficiently recovered by decorrelation using a Fast Walsh Transform (FWT). Methods and apparatus for performing an FWT are described in U.S. Pat. No. 5,357,454 to Dent for "Fast Walsh Transform Processor", which is expressly incorporated here by reference. Walsh-Hadamard sequences have structural properties that make correlation of a received signal with candidate Walsh-Hadamard sequences possible to do with much less complexity than brute force correlations. The results of an FWT operation are substantially identical to correlating the received sequence with all Walsh-Hadamard sequences of a given length. The correlation of one received length-M sequence with a bank of M length-M candidate sequences generally requires on the order of M.sup.2 operations. Using Walsh-Hadamard sequences, the correlation of a received sequence requires only on the order of M.multidot.log.sub.2 M operations since the FWT can be utilized.
It is desirable to provide various types of communication services to meet various consumer demands, such as voice telephony, facsimile, e-mail, video, Internet access, etc. Moreover, it is expected that users may wish to access different types of services at the same time. For example, a video conference between two users would involve both voice and video support. Some services require higher data rates than others, and some services would benefit from a data rate that can vary during the communication.
Varying the spreading factor is a known technique for accommodating variable data rates in spread spectrum communication systems. This and other CDMA communication techniques are described in U.S. patent application Ser. No. 08/890,793 filed by F. Ovesjo et al on Jul. 11, 1997, for "Channelization Code Allocation for Radio Communication Systems", which is incorporated here by reference, and in U.S. Pat. No. 5,751,761 to Gilhousen. As mentioned above, a DS-CDMA spread spectrum system spreads a data signal across an available bandwidth by multiplying the data signal by spreading sequences. By varying the number of chips per data symbol, i.e., by varying the spreading factor, while keeping the transmitted chip rate fixed, the effective data rate can be controllably varied. It will be understood that the data rate, or channel bandwidth, is determined, at least in part, by the spreading sequence's length M, i.e., the spreading factor applied to the data (information bits).
In typical implementations of the variable spreading factor approach, the spreading factor is limited by the relationship to SF=2.sup.k .times.SF.sub.min where SF.sub.min is the minimum allowed spreading factor corresponding to the highest allowed user rate. In currently proposed WCDMA communication systems, the spreading factor can be one of a number of predetermined values, e.g., 256, 128, 64, or 32, that correspond to channel bit rates of 16, 32, 64, and 128 kbps, respectively.
These variable spreading factors can be provided by respective subsequences of a family of Walsh-Hadamard sequences. These orthogonal variable spreading factor (OVSF) sequences can preserve the orthogonality between channels of different bit rates 30 and spreading factors, and they can conveniently be organized in a tree structure. This is described in Section 6.2.1 of UTRA FDD, Spreading Modulation and Description, UMTS (xx.05) v0.1.0, ETSI Secretariat (Sep. 1998), and in U.S. Pat. No. 5,751,761 cited above.
FIG. 4 depicts a typical tree structure for Walsh-Hadamard sequences, or codes. Levels in the code tree define channelization codes of different lengths, corresponding to different spreading factors. In FIG. 4, the root of the tree is indicated by code C.sub.1,1 that has a spreading factor SF=1, level 1 of the tree includes codes C.sub.2,1 and C.sub.2,2 that each have spreading factors of 2, and so forth. At each level, exemplary corresponding sequences, or codes, are indicated. For the root level, the example shown is [1], for level 1, the example codes shown are [1 1] and [1 -1], and so forth. In the notation C.sub.k,i illustrated, k is the spreading factor SF and the index i simply distinguishes codes at the same level. It will be appreciated that the tree continues to branch as one moves to the right in FIG. 4 and that it is not necessary for the code sequence at the root level to have only one element as illustrated.
All codes in a code tree cannot be used simultaneously in the same cell or other environment susceptible to mutual interference because all codes are not mutually orthogonal; a code can be used if and only if no other code on the path from the specific code to the root of the tree or in the sub-tree below the specific code is used. This means that the number of available channelization codes is not fixed but depends on the rate and spreading factor of each channel in the group of channels that potentially can mutually interfere.
Eligible channelization codes can be allocated randomly from the available eligible codes in the code tree structure for channels of different rates and spreading factors, which is to say that the eligible codes may be allocated without co-ordination between different connections, other than maintaining orthogonality. On the uplink, different users (connections) use different scrambling codes, so all of the spreading codes in a tree can be used for each user without co-ordination among different users. The situation on the downlink could be different because the BS typically uses only one scrambling code for all users (connections). Thus, spreading codes cannot be allocated so freely; co-ordination among users is needed.
The random allocation of codes from a tree results in an uneven distribution in the tree of the codes allocated in a cell. This limits the use of certain codes due to constraints described above, thus resulting in a higher incidence of blocking and/or delay for new calls. One possible solution is to re-arrange the codes allocated to ongoing calls, making codes available for new calls. The drawback of this strategy is that a large number of re-arrangements can be required, rendering this strategy difficult to use due to heavy signaling overheads involved.